Noise is a big source of concern in today's society. Various types of machines emit sounds of various frequencies. Certain frequencies having a certain sound pressure level are considered noise and must due to for instance regulations be removed or reduced to below an allowable sound pressure level.
One example of such a machine is any type of engine, pump or similar emitting noise which cannot be dampened or absorbed with standard sound absorbing or noise reducing means, such as resonators or insulation.
A vehicle engine running at a specific engine rpm causes a rotary equipment such as a supercharger, fan, pump, generator, compressor or other similar equipment to emit a specific noise frequency proportional to the rotational frequency of the rotary equipment or rotary equipment rpm. The engine itself can also be seen as a rotary equipment, for instance in the case of electric motors. As the rotational frequency of the rotary equipment is coupled to the engine rpm a specific noise frequency having a frequency being determined by the engine rpm is emitted. The specific noise frequency is thus determined by engine input conditions such as rpm and/or other factors. Such a specific noise frequency usually has an amplitude which causes the specific noise frequency to be easily distinguishable over the ambient broadband sound originating from for instance the combustion engine, wheels and wind.
It is not always possible to absorb or reduce these specific noise frequencies by ordinary means such as resonators or insulation. If the specific noise frequency is of a low frequency, i.e. below approximately 300 Hz, active noise control can be utilized and a signal having the same frequency as the specific noise frequency can be output out of phase with the specific noise frequency, thereby cancelling the specific frequency. However, this is not possible for specific noise frequencies over certain frequencies, e.g. above approximately 300 Hz.
There is thus a need for an improved method of dealing with unwanted specific noise frequencies, especially for specific noise frequencies which cannot be cancelled by active noise control or ordinary forms of damping or insulation.